The present invention provides an improved process for the preparation of alkyl glyoxylates by the oxidative dehydrogenation of the corresponding alkyl glycolate.
U.S. Pat. No. 1,614,195 and European Patent No. 0149439 disclose processes involving the gas phase reaction of alkyl glycolates with an oxygen source in the presence of a catalyst. A suitable oxygen source for oxidizing the alkyl glycolate is air. A number of catalysts may be used to promote the reaction. European Patent No. 0149439 discloses carrying out the reaction in the presence of a metallic silver catalyst at temperatures ranging between 325.degree.-685.degree. C. The details of this reaction, including the reactants and other process conditions are provided in European Patent No. 0149439.
The oxidative dehydrogenation of alkyl glycolates is strongly exothermic, resulting in temperatures in excess of 700.degree. C. if air is used as the oxygen source unless measures are taken to limit the reaction temperature. Temperatures greater than 700.degree. C. are known to decompose the reaction constituents. Also, high temperatures and the presence of metallic materials that catalyze undesirable reactions are the major factors contributing to a reduction in yield. Thus far, temperature control in these exothermic reactions has been achieved either by removal of heat through the wall of the reactor, by dilution of the reactant stream with a large volume of an inert substance, e.g., nitrogen, to provide additional heat capacity, or by conveying the reactants at such a high velocity through the reactor that contact time at the high temperature is minimized.
Each of these techniques for controlling temperature has certain drawbacks. Where the temperature is controlled by the removal of heat through the reactor walls, it has been necessary to use small reactors with a high ratio of surface area to volume to effect sufficient heat transfer through the reactor wall. This makes large-scale operations economically impractical. Most of the early laboratory processes in the prior art utilize this concept. For example, the reactor described in U.S. Pat. No. 1,614,195 is a tube having an inside diameter of from 2-4 cm and a length of 10M. This reactor controls the temperature of the reaction by means of heat losses through the wall of the reactor. Small diameter reactors of this type, which are designed for cooling, i.e., having a relative large surface area/volume, are expensive, especially where a relatively large throughput or production is required.
Where the temperature is controlled by dilution of the reactant stream, the resultant large volumes of the dilute reactant stream must be handled with the consequent penalty in large equipment size and lower catalyst effectiveness. Also, isolation of the desired glyoxylate from the dilute process stream is more difficult and expensive. For example, U.S. Pat. No. 4,340,748 discloses as the preferred embodiment using from 40-60 moles of nitrogen carrier gas/mole of glycolic acid ester to dilute the reaction mixture and carry away the heat of reaction. The use of such large volumes of inert gas which serve as a heat sink presents additional problems in handling of large volumes of gases and the separation of the gaseous reaction mixture from the inert gases. Further, catalyst productivity, measured as space/time yield, is low due to the high degree of dilution.
Where the temperature is controlled by using very high velocities through the catalyst bed, the pressure drop through the catalyst bed becomes excessive, necessitating the use of large equipment. Another disadvantage is that with the short contact time, a high conversion of the alkyl glycolate is not obtained; consequently, isolation of the desired product from unreacted starting material becomes more difficult. For example, European Patent No. 149,439 teaches a method of producing alkyl glyoxylates in which extremely high gas velocities and corresponding short contact times are employed. This procedure limits the conversion of the glycolate to less than 70 percent, causes a high pressure drop in the reactor, and downstream difficulties in separating product from unreacted starting material.
The prior art processes for the preparation of alkyl glyoxylates by the catalytic dehydrogenation of alkyl glycolates use a single reactor. U.S. Pat. No. 4,537,726 discloses a method for the preparation of isocyanates by reacting formamides with an oxygen-containing gas in the presence of silver or silver combination catalyst in at least two essentially adiabatic reactors in series. The technology of U.S. Pat. No. 4,537,726 cannot be translated to the oxidative dehydrogenation of alkyl glycolates due to the difference in the reactions.
U.S. Pat. No. 4,343,954 discloses the use of a two-stage adiabatic reactor system for production of formaldehyde, in which one reactor contains a silver catalyst and the other, a metal oxide catalyst. The effluent from the first reactor is cooled and fed to a second adiabatic reactor through a metal oxide catalyst bed. The conversion of methanol to formaldehyde over a silver catalyst by two separate and simultaneous pathways is detailed in Kirk-Othmer Encyclopedia of Chemical Technology (3rd ed., Vol. 11, p. 237). The oxidative dehydrogenation leading to formaldehyde is strongly exothermic and would result in very high temperatures if it were the only process involved. However, a parallel dehydrogenation pathway to formaldehyde is strongly endothermic and provides an internal heat sink to lower the temperature inside the reactor.
The technology of U.S. Pat. No. 4,343,954 cannot be translated to the oxidative dehydrogenation of alkyl glycolates, since the endothermic dehydrogenation pathway is unimportant in this case, accounting for less than 10 percent of the glyoxylate formed. Since the concurrent endothermic reaction is less prevalent in the alkyl glyoxylate reaction, the temperature during the oxidative dehydrogenation reaction would become excessive if the technology of U.S. Pat. No. 4,343,954 were practiced. Furthermore, the glyoxylate product possesses a second carbonyl group adjacent to the aldehyde function and, consequently, is less stable than formaldehyde. Excessive temperatures cause the glyoxylate to release one of the two adjacent carboxyl groups as a mole of carbon monoxide.